Timothy W. Stewart

Quantifying mechanisms for zebra mussel effects on benthic macroinvertebrates: organic matter production and shell-generated habitat.

Quantitative descriptions of interactions between zebra mussels (Dreissena polymorpha) and other organisms are needed for an understanding of zebra mussel effects on energy flow and community dynamics in North American freshwater ecosystems. We used a field experiment to quantify effects of 2 potential mechanisms for increasing benthic macroinvertebrate biomass and densities in the Great Lakes following the zebra mussel invasion: 1) habitat created by zebra mussel shells, and 2) habitat and food provided by increases in benthic organic matter. Macroinvertebrate biomass, macroinvertebrate densities, and organic matter mass (hereafter termed organic matter) were measured on substrates (tiles without zebra mussels, with zebra mussel shells, and with live zebra mussels) that were held in western Lake Erie for 37 d. Organic matter was 3×-5× greater on tiles with live mussels than on plain tiles or tiles with shells, demonstrating that filter-feeding zebra mussels increased organic matter on benthic substrates. Organic matter did not differ on tiles with shells and tiles without mussels. Similarly, total macroinvertebrate biomass was 2×-5× greater on tiles with live zebra mussels than on tiles with shells and tiles without mussels; however, macroinvertebrate biomass also was 3X greater on tiles with shells than on tiles without mussels. Habitat created by shells was the most important cause for zebra mussel-generated increases in total macroinvertebrate biomass and densities of Hydridae, Dugesia tigrina (Turbellaria), Physella sp. (Gastropoda), Gammarus fasciatus (Amphipoda), and Microtendipes pedellus (Chironomidae). However, organic matter produced by zebra mussels also caused increases in total macroinvertebrate biomass and densities of Physella and Microtendipes. Zebra mussels did not reduce densities of any macroinvertebrate taxon. Because benthic macroinvertebrates are important prey for crayfish and benthivorous fishes, increases in macroinvertebrate abundance may be important in channeling energy from zebra mussels to higher trophic levels in the Great Lakes.

Macroinvertebrate communities and environmental conditions in recently constructed wetlands

We quantified macroinvertebrate community characteristics in nine temporary or permanent wetlands, and related these to environmental conditions. Macroinvertebrates inhabiting the water column and shallow sediment (42-cm depth) were sampled 20 months after wetland construction in June 2005. A total of 29 taxa were collected, and macroinvertebrate communities varied among wetlands. Total macroinvertebrate biomass (mean ± SE = 16.44 ± 4.72 g AFDW/m3) and densities (mean ± SE =372,096 ± 124,972 individuals/m3) were positively related to coarse particulate organic matter abundance (living and nonliving plant matter; CPOM) and negatively related to turbidity. Density of ecologically sensitive EOT (Ephemeroptera, Odonata, Trichoptera) taxa was also positively related to CPOM and negatively related to turbidity. Total taxa richness was negatively related to turbidity, and percent of total macroinvertebrate density consisting of EOT (% EOT) was positively related to CPOM. These relationships were greatly influenced by 10 dominant taxa (nematodes, physid snails, mites, small squaregill mayflies, narrowwinged damselflies, biting midges, non-biting midges, ostracods, cladocerans, and cyclopoid copepods) that were positively associated with CPOM and negatively related to turbidity. Two wetlands inhabited by common carp (Cyprinus carpio) appeared to be in the poorest condition. These wetlands had the lowest macroinvertebrate biomass and densities and highest turbidity. Additionally, although net uptake of total nitrogen (TN) occurred in these high-turbidity wetlands, NH3 concentrations were two-fold higher in outflow than inflow. Net uptake of total phosphorus (TP) occurred only in wetlands with low turbidity, high CPOM abundance, and high macroinvertebrate abundance and diversity. To enhance macroinvertebrate abundance and diversity and ecological functions (e.g., nutrient removal) in newly constructed wetlands, management efforts should be directed toward increasing plant abundance and reducing turbidity.

Dreissena-Shell Habitat and Antipredator Behavior: Combined Effects on Survivorship of Snails Co-Occurring with Molluscivorous Fish

Prey employ various strategies to avoid predators, including use of structurally complex habitat where predator foraging efficiency can be reduced. We used laboratory experiments to quantify combined effects of antipredator behavior and habitat provided by zebra mussel (Dreissena polymorpha) shells on survivorship of snails (Physella heterostropha) co-occurring with molluscivorous fish. First we determined predator effects on prey habitat use in aquaria with and without Dreissena-shell beds (i.e., ceramic tiles with and without shells attached to them, respectively) by comparing snail distribution in the absence of fish and in the presence of chemical cues produced from redear sunfish (Lepomis microlophus) preying on snails. Effectiveness of shell beds and other habitats as refuges from actual predation were then evaluated by introducing fish to aquaria that previously received chemical cues, and recording numbers of snails remaining in various habitats 24 h later. Shells and fish had strong independent and interactive effects on snail habitat use. A greater % of snails inhabited tiles when shells were present than when they were absent, and this occurred regardless of the absence or perceived presence of fish. However, fish cues caused snails to increase use of shell beds. In an additional experiment, we found that snails under predation risk also altered their microdistribution within shell beds by abandoning exposed shell surfaces in favor of interstices between shells where they were less vulnerable to fish. The combination of protective shell habitat and antipredator behavior caused snail survivorship to be 8-fold greater in the treatment with shells and fish than in the same predator treatment without shells. Our results suggest that avoidance of predators through use of zebra mussel beds is an important mechanism for the positive Dreissena effects on co-occurring benthic invertebrates that have been observed in field studies.

Assemblage and Population-Level Responses of Stream Fish to Riparian Buffers at Multiple Spatial Scales

Abstract Riparian buffers can improve stream water and habitat quality by reducing non-point-source pollution (e.g., nutrients and sediment), increasing canopy cover and thereby reducing water temperature, and contributing allochthonous organic matter (e.g., leaf litter and woody debris). However, the influence of riparian buffers on biotic assemblages in streams is poorly understood, particularly in the Midwestern United States. In this study, we evaluated the effects of riparian buffers on instream habitat, fish assemblage structure, and population characteristics (i.e., the growth of two small-bodied species) in three streams in central Iowa. The streams were surveyed at two spatial scales; specifically, 41 reaches were sampled and 247 macrohabitat types (i.e., pool, riffle, and run) were subsampled in the summer of 2007. Fish assemblage structure data were summarized into separate data sets by the relative abundance of individual species (i.e., fish per minute of electrofishing) and guilds (e.g., trop...

Linear and Nonlinear Effects of Habitat Structure on Composition and Abundance in the Macroinvertebrate Community of a Large River

Abstract We used an experiment and regression analyses to quantify effects of spatial variation in habitat structure abundance on a riverine macroinvertebrate community under winter conditions. Concrete slabs (0.21 m2; n = 24) with different numbers of stones (mean individual stone surface area = 6.44 cm2) attached to upper faces were placed in the James River and retrieved after 28 d. Macroinvertebrate abundance and taxonomic richness on slabs were significantly positively related to stone abundance. Total macroinvertebrate abundance and abundance of oligochaetes (Nais spp.), Asiatic clams (Corbicula fluminea), caddisflies (Leptoceridae), riffle beetles (Elmidae) and stoneflies (Strophopteryx sp.) were linearly related to stone abundance. However, nonlinear relationships occurred between stone abundance and macroinvertebrate taxonomic richness and between stone abundance and abundance of dragonflies (Erpetogomphus sp.), caddisflies (Hydropsychidae), chironomids (Eukiefferiella spp.), mayflies (Ephemerellidae) and stoneflies (Taeniopteryx sp.). Nonlinear relationships were usually characterized by dramatic increases in macroinvertebrate abundance and taxonomic richness across a gradient of increasing stone abundance when abundance was low (0–43 stones/0.08 m2 slab face; 0–40% of slab face covered by stones), but weak responses to additional stones at higher stone abundance (84–160 stones/0.08 m2; 89–96% cover). These nonlinear relationships reflected similar nonlinear relationships between abundance of stones and particulate matter. We conclude that small quantities of habitat structure have significant positive effects on macroinvertebrate abundance and diversity in the James River during winter. At a local scale, habitat structure promotes macroinvertebrate colonization and retention by increasing habitat diversity. Stones and similar physical objects also indirectly benefit macroinvertebrates by trapping particulate matter that provides animals with food and additional habitat.

Land Use, Stream Habitat and Benthic Invertebrate Assemblages in a Highly Altered Iowa Watershed

Abstract Knowledge of relationships between land cover (i.e., land use) and abiotic and biotic features of headwater streams enhances our ability to predict and effectively assess conditions in a variety of aquatic ecosystems. We evaluated land use effects on stream condition in an Iowa watershed dominated by intensive row crop agriculture and low- intensity urban development by quantifying relationships among land cover, stream invertebrate assemblages and other stream biophysical characteristics (i.e., invertebrate habitat) at 29 sites. On average, 81% of subbasin land cover was agricultural and 6% of land cover was urban across study sites. High nitrate concentrations (range  =  5.6–29.0 mg/L) and high relative abundance of oligochaetes and chironomid midges reflected degraded conditions at all sites. However, agriculture and urban land use appeared to have different effects on stream features. Nitrate concentrations were positively related to agricultural land cover, and turbidity and nitrate concentrations were negatively related to urban land cover (P ≤ 0.05). Invertebrate densities and taxonomic diversity (i.e., total taxa richness, % EPT) were also positively related to agricultural land cover and negatively related to urban land cover. Regardless of land use, highest invertebrate abundance and taxonomic diversity occurred at sites with abundant coarse particulate organic matter, plants and coarse inorganic substrate. Relationships between land cover and invertebrate variables were strong at both local and subbasin measurement scales. Based on invertebrate assemblages, which integrate multiple instream features, we conclude that urban land use had greater adverse effect on stream condition than agriculture in our study watershed. Although impacts of urbanization on stream invertebrates frequently exceed effects of agriculture, this has not previously been demonstrated in Iowa or other Midwestern landscapes so heavily dominated by agriculture.

A Field Experiment to Determine Dreissena and Predator Effects on Zoobenthos in a Nearshore, Rocky Habitat of Western Lake Erie

Results from several previous studies showed that zebra and quagga mussels (Dreissena polymorpha and D. bugensis, respectively) are key structuring agents of benthic invertebrate communities. However, the relative effects of Dreissena and other potentially important biotic factors on these communities remain poorly understood. The goal of this study was to improve our understanding of community-structuring mechanisms in habitats with Dreissena by quantifying simultaneous effects of Dreissena and large benthic predators (primarily fish) on invertebrates inhabiting hard substrates in western Lake Erie. Biomass, densities, and diversity of invertebrates were compared on bricks with low and high (=ambient) Dreissena biomass that were held in western Lake Erie for 49 d within cages that excluded large predators, partial cages where predators had access to invertebrates, and cageless-reference plots. Dreissena had positive effects on total invertebrate biomass, and biomass of gastropods, chironomid larvae, and the amphipod Echinogammarus ischnus. Dreissena also caused increases in taxonomic richness of invertebrates, Echinogammarus body size, and densities of several invertebrate taxa. Our results suggest that predators reduced Echinogammarus biomass in the high-Dreissena biomass treatment. However, predators did not affect Dreissena biomass, total invertebrate biomass, invertebrate densities, or taxonomic richness of invertebrates. We conclude that both Dreissena and large predators regulate invertebrate community structure on hard substrates in western Lake Erie, but that Dreissena are of much greater importance.

Learning about restoration of urban ecosystems: a case study integrating public participation, stormwater management, and ecological research

Restoration of ecosystem functions in urban environments is made challenging by 1) a public that often lacks understanding of ecological principles, 2) inadequate evidence of the effectiveness of restoration practices, and 3) difficulty integrating social and biophysical factors in studies of urban ecosystems. This paper describes a case study in which potential solutions to these challenges were explored. We facilitated collaborative learning through public participation in the design and implementation of an urban riparian buffer along a headwater stream in a neighborhood park, a process that was informed by ecological research. Learning outcomes were evaluated using surveys and qualitative assessment of discussion. Results indicated that participants’ knowledge about water quality problems associated with urbanization, stormwater, and nonpoint-source pollution increased, familiarity with stormwater management practices increased, and perceptions about the importance of stream ecosystem functions changed. In-stream monitoring of sediment delivery, as well as direct measurements of buffer infiltration capacity, provided early evidence of buffer effectiveness in prevention of sediment inputs to the stream and absorption of runoff from surrounding surfaces. This study provides a useful model for integration of collaborative learning through participation, ecological restoration, and ecological research in an urban setting. Elements deemed essential to success of this model included an opportunity for dialog focused on a specific natural feature, sustained interaction between participants and researchers, opportunities for hands-on participation by urban residents, and flexibility in restoration practice installation.

Macroinvertebrate Assemblages in Iowa Prairie Pothole Wetlands and Relation to Environmental Features

Macroinvertebrate assemblage characteristics and other biophysical features were measured in the open-water zone of 17 semipermanent/permanent prairie pothole wetlands in Iowa. Relations among variables were used to identify likely determinants of invertebrate abundance and diversity, and useful indicators of wetland condition (i.e., health). Abundant invertebrates included physid and planorbid snails, oligochaetes, leeches, caenid mayflies, corixid bugs, chironomid midges, and amphipods. Abundant taxa were distributed across multiple functional feeding and habit groups. Invertebrate densities and taxonomic diversity were positively associated with plant/coarse particulate organic matter (plant/CPOM) abundance, and negatively related to turbidity, water-column concentrations of nitrogen and phosphorus, and presence of large-bodied fishes. Results supported hypotheses that plant/CPOM was an important invertebrate resource, and that plant/CPOM abundance was reduced in turbid, hypereutrophic wetlands. Fish likely reduced invertebrate abundance and diversity through predation and/or cascading effects on plants, turbidity, and nutrients. Wetlands with high invertebrate abundance and diversity, abundant plant/CPOM, low turbidity and nutrient concentrations, presence of tiger salamanders (Ambystoma tigrinum), and absence of large-bodied fishes appeared to be in the best condition. These features should be routinely measured to effectively assess condition of Iowa prairie pothole wetlands. Additionally, eliminating non-native fish populations may be critical to improving condition of these wetlands.

Environmental Factors Causing Local Variation in Density and Biomass of the Snail Leptoxis Carinata, in Fishpond Creek, Virginia

Abstract Snails in the family Pleuroceridae have strong effects on community structure and energy and nutrient pathways in many North American rivers and streams. Identifying factors determining densities and biomass of pleurocerids is, therefore, an important step toward predicting the relative importance of these snails in an ecosystem. We used plot sampling to identify environmental factors causing variation in demographic features of the pleurocerid snail Leptoxis carinata in a 3rd order stream in south central Virginia. Snails were collected from forty 0.22 m2 plots located within a 165 m segment of Fishpond Creek and L. carinata density, mean shell width, mean body mass and total biomass in each plot were measured. Measurements were also taken for several environmental variables. Multiple regression indicated that topographic complexity of substratum was the primary cause of variation in L. carinata density and total biomass among plots. Snail density and total biomass were both greater in topographically complex plots with large substratum particles (i.e., boulders, cobble) that likely provide refuge from high flow events and are preferred habitat for foraging. Temporal changes in several demographic features were also quantified. Mean shell width and body mass of individual snails increased during the 28 d study (12 March 2001–8 April 2001), but total biomass declined following a flood that occurred from 25 March 2001–5 April 2001.